Why is my seawater hypochlorite generator not producing the concentration I want? The short answer to the that question is to stop worrying about concentration (how strong the hypochlorite is) and instead understand your plant’s specified generating capacity, which is typically rated as the mass of hypochlorite generated over a period of time. This is typically measured in kg/hour or lbs/day generated.
What you really need to worry about is the free residual value of oxidizer in the parts of your cooling water system that have the longest residence time – the points in the system where treated water takes a long time to flow from the hypochlorite injection point to the measurement point. I doubt that anybody ever bought a seawater hypochlorite generator because they wanted to make chemicals. Sure, it’s important to be certain the system is outputting what it’s rated to produce, and that it is doing so efficiently. Typically, our clients hope to achieve a single outcome by purchasing a hypochlorite generator: to use the oxidizer produced to prevent biofouling from destroying their cooling water system.
If adequate residual quantities of free oxidizer are not present at all points in the cooling water system, the concentration of hypochlorite produced by the electro chlorinator (hypochlorite generator, EC package, etc.) really doesn’t matter. Biofouling agents – materials like barnacles, mussels, tubeworms and corrosion-causing bacteria – will be free to grow, and your system will be negatively affected by them.
Now, for those of you who still need to understand why the concentration (how strong the hypochlorite is) varies all the time, here is more than you ever wanted to know about the subject.
8 major factors that affect concentration and general hypochlorite production:
1) Driving amperage
2) Seawater flow through the cells
3) Salinity of the seawater
4) Temperature of the seawater
5) Sampling errors
6) Testing errors
7) Cell fouling
8) Coating loss
1.) Driving amperage
Driving amperage is the most important factor in hypochlorite production. This is because the electrochemical process creating the sodium hypochlorite is driven by the flow of electrons and the associated half-cell reactions at the anode and cathode creating chlorine, amongst other elements. The higher the amperage, the more electrons flow to the cells; the more electrons, the more chlorine is generated. The amperage can and will vary slightly due to a number of factors, but as the amperage increases the hypochlorite production does, too. As it comes down, so goes the production. If the production goes up (kg/h) and the flow remains constant, concentration will increase.
2.) Seawater Flow
If amperage is king of concentration, seawater flow is the crown prince. The concentration looks like this: XXX mg/l NaOCL. Well, if I produce a given weight of hypochlorite in milligrams, but add it to twice the volume of seawater in litres, well, low and behold! The concentration (how strong the solution is) will automatically be cut in half. This is a perfect example of why the statement above “don’t worry about concentration”, and why weight/time is so important to understand. Small variations in seawater flow through the cell are guaranteed, as flow cannot be exactly controlled, and the resulting concentration will vary in lockstep with this. The governing equation used to determine the target concentration based upon flow is (rated productive capacity in kg/hr*1000)/(seawater flow in m3/hr) = expected concentration in mg/L. Please note that to convert ppm to mg/L, the specific gravity of the seawater stream must be considered using the equation ppm*specific gravity = mg/L.
3.) Salinity of the seawater
The salinity of the seawater should not vary much. However, if it does vary – due, for instance, to tidal factors or shifts in currents or runoff in the spring – so will the plant’s production. (To see a chart of efficiency versus salinity, go to our website and sign up for the members-only section, where you will gain access to a wide array of helpful troubleshooting guides applicable to any hypochlorite generator.) An increase salinity will result in a decrease in the voltage required to drive a given current. The opposite is also true for decreased salinity. In extreme cases of low salinity, the system will typically produce an alarm indicating that, due to hardware limitations, adequate voltage is not available to drive the desired current.
Temperature is another factor that shouldn’t vary much. If this is not the case (due to cell blockage or low flow through the cell, for instance), the chlorine efficiency will be greatly reduced. Also note that, in the same way that salinity can affect voltage, so can temperature. Though this rarely affects productive capacity, it’s something to keep in mind for extreme operating cases using low-temperature feed water.
5.) Sampling Errors
Sampling errors are easy to make, but also easy to correct. The most common sampling error is not allowing solution to adequately flush the line before taking the sample. Allow the sample line to flush for an adequate and consistent period before collection – typically 2.5x the volume of the sample line. This will prevent solution that has remained stagnant in the line for long periods to flush out and will ensure the reading is taken using the current set of parameters on the system being tested. When sampling from a tank, be sure to allow the tank to undergo at least two volume exchanges before sampling.
6. Testing Errors
Testing errors are also easy to make. The biggest testing error is not in the testing process, although here the procedures must be followed exactly. The biggest testing error is not taking enough tests. The concentration will not be a constant number, which is why plants are designed for an average weight per hour. You need to take several (at least four) samples over the course of an hour, and average the results to get a real picture of plant output. This will also allow the tester to identify any outliers in the data generated by the analysis method.
7.) Cell Fouling
Cell fouling is caused by the buildup of hydroxide scales on the cathode. Regular cleaning of the cathode will dissolve and flush out this scale. If cleanings are not performed, or are not frequent enough, scale can build up and prevent the electrochemical process from happening in that spot. If not cleaned for long periods, this hydroxide scale can accumulate to the point that it causes mechanical failure or a low flow that overheats the cell, causing irreparable damage. In extreme cases of neglect, hydroxide scales can bridge the anode and electrode and cause the cells to fail. Again, regular cleaning is extremely important.
8.) Coating Loss
Coating loss is a normal part of the operation. Typically, cell coatings are designed to provide a five- year life. Once this coating wears off, portions of the cell will become sacrificial and dissolve away, reducing system output and eventually causing the system to fail altogether.
Hypochlorite Generator Service
Hopefully this article has helped you understand the top 8 factors affecting the concentration produced by your seawater hypochlorite generation plant. If you would like access to helpful charts and troubleshooting procedures applicable to any brand of seawater hypochlorite generator, or would like some help with troubleshooting your system, please visit our website at www.h2oinc.com.